Quality assurance in radiotherapy by in vivo dosimetry. 1. Entrance dose measurements, a reliable procedure.

Entrance dose measurements were performed with semiconductor detectors on patients treated for head and neck and brain tumors with a 6 MV X-ray beam. A total number of 554 treatment set-ups were measured. The results showed a gaussian distribution with a mean value of 97.8% and a standard deviation of 2.8%. A systematic error of 2.2% on the mean value was shown to be due to a systematic deficiency in the algorithm used in the planning system and to a systematic error in the application of the dosimetry protocol. Two treatment techniques were identified leading to an erroneous dose delivery. Finally, large deviations (more than 2 S.D.) of the measured dose from the expected dose were detected in 3% of the measured treatment set-ups, the sources of the errors could in all cases be identified and eliminated in the further treatment sessions. This study demonstrated the reliability of the use of semiconductor detectors for in vivo dosimetry and its usefulness as part of a departmental quality assurance program.